<html>
<head>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    body {
      width: 200px;
      height: 100px;
    }

    div {
      line-height: 100px;
      font-size: 42px;
      text-align: center;
    }
  </style>
</head>
<body>
<div id="clock_div"></div>
<script src="my_clock.js"></script>
</body>
</html>